The present invention is directed toward optical filters, and in particular to optical filters having a center wavelength that is substantially independent of temperature.
Optical interference filters rely on principles of interference that modify reflected intensities of light incident on a surface. A familiar example of interference is the colors created when light reflects from a thin layer of oil floating on water. Briefly stated, by modifying the interface of a substance and its environment with a third material, reflectivity of the substance can be significantly altered. This principle is used in the fabrication of optical interference filters. These filters can be used as one of, or as the main filtering element in optical add/drop multiplexers, for example, employed in optical communication systems to select one or more channels from a transmission signal.
In its most simple form, an optical interference filter includes a reflective layer provided on a substrate. The reflection layer includes a cavity including two partial reflectors or mirror layers separated by a spacer. Each partial reflector, also referred to as a quarter-wave stack, includes alternating layers of high and low refractive index dielectric materials. Each of these layers has an optical thickness (defined as: physical thickness x refractive index) of a quarter-wavelength (xcex/4), where xcex is a xe2x80x9ccenter wavelengthxe2x80x9d, i.e., the wavelength of light to be transmitted by the filter. The spacer typically further includes at least one half-wave (or multiple half-wave) layer, and thus typically has twice the thickness of an individual quarter-wave layer. By appropriate choice of dielectric materials, the interference filter can be designed to transmit optical signals within a relatively narrow band about xcex through the reflection layer and the substrate, while wavelengths outside the band are reflected. Typically, many cavities are provided on a substrate.
A filter that reflects a desired wavelength while transmitting other wavelengths can also be constructed by forming a reflective layer with one or more cavities having appropriately selected thicknesses of the quarter and half wave layers.
Filters are typically manufactured by sputtering, for example, the quarter and half wave dielectric layers onto the substrate. The substrate must therefore have good adhesion to these dielectric layers. Moreover, since the filters are often critical components in telecommunications equipment and are frequently placed in harsh environments, the substrate should preferably have good mechanical strength as well as excellent chemical and atmospheric resistance. The substrates should also transmit light with little loss, and have a refractive index of 1.5 to 1.6 for use in optical communications systems carrying signals at wavelengths in the range of 1270 to 1670 nm. Further, the substrate should provide an effective coefficient of thermal expansion in the range of 9-13 ppm/Cxc2x0. If Young""s Modulus and Poisson""s ratio are then taken into account, the filter will have relatively low wavelength dependence.
Many substrates are commercially available for incorporation into optical filters, but lack one or more of the criteria identified above. In particular, many commercially available substrates degrade or lose adhesion when exposed to relatively high temperatures and humidities. Also, conventional filter designs have failed to adequately account for temperature-dependant wavelength variation. Single-sided substrate filters, when bonded to another element such as a Grin lens, can also lose their temperature independence by virtue of a mismatch in the thermal expansion between the filter and the Grin lens.
Consistent with the present invention, an optical filter is provided comprising a single crystal or crystalline quartz (hereinafter referred to as xe2x80x9cquartzxe2x80x9d) substrate and a reflective layer provided on the quartz substrate. According to another aspect of the invention, an optical filter is provided comprising a reflective layer provided between first and second substrates. In either arrangement, the reflective layer has an associated center wavelength, which is substantially independent of temperature.